The extreme host specificity of pathogenic neisseriae limits investigations aimed at the analysis of bacterialhost interactions almost completely to the use of in vitro models. Although permanent epithelial and endothelial cell lines are already indispensable tools with respect to initial infection processes, studies concerning the interaction of neisseriae with phagocytic cells have been confined to primary human blood cells. We investigated the use of human leukemia-derived monocytic and myelomonocytic cell lines that can be differentiated in vitro towards phagocytic cells by a panel of chemical and biological reagents including cytokines, vitamin analogs, and antileukemia drugs. Whereas tumor necrosis factor alpha, gamma interferon, bufalin, or granulocyte-macrophage colony-stimulating factor only marginally increased the ability of monocytic Mono-Mac-6 and myelomonocytic JOSK-M cells to interact with the bacteria, retinoic acid and vitamin D3 treatment for 2 to 4 days led to highly phagocytic cells that internalized gonococci in an Opa protein-specific manner. This is comparable to the phagocytosis by primary monocytes from human blood, where more than 80% of cells are infected with intracellular bacteria. The increased phagocytic activity of JOSK-M cells following in vitro differentiation was paralleled by enhanced oxidative burst capacity. Whereas undifferentiated cells responded to neither phorbol 12-myristate 13-acetate nor other known soluble and particulate stimuli, cells incubated with retinoic acid and bufalin showed the same pattern and the same intensity of oxidative burst activity in response to Neisseria gonorrhoeae as primary cells: Opa-expressing gonococci elicited an oxidative burst, whereas Opa2 gonococci did not. The surface expression of major histocompatibility complex (MHC) class II molecules was only slightly changed after retinoic acid treatment. Also, phagocytosis of gonococci had no influence on MHC class II surface expression. Taken together, our results demonstrate that in vitro-differentiated human myelomonocytic JOSK-M cells provide a suitable model for the study of a variety of aspects of the gonococcal interaction with phagocytes.